Low cost, high average power, high brightness solid state laser

ABSTRACT

A high average power, high brightness solid state laser system. We first produce seed laser beam with a short pulse duration and frequency in excess of 1,000 pulses per second. A laser amplifier amplifies the seed pulse beam to produce an amplified pulse laser beam which is focused to produce pulses with brightness levels in excess of 10 11  Watts/cm 2 . Preferred embodiments produce an amplified pulse laser beam having an average power in the range of 1 kW, an average pulse frequency of 12,000 pulses per second with pulses having brightness levels in excess of 10 14  Watts/cm 2  at a 20 μm diameter spot which is steered rapidly to simulate a larger spot size. These beams are useful in producing X-ray sources for lithography. 
     In one preferred embodiment, the seed beam is produced in a mode locked Nd:YAG oscillator pumped by a diode array with the frequency of the pulses being reduced by an electro-optic modulator. In a second preferred embodiment, the seed beam is Q switched and includes a Pockels cell for cavity dumping. In a third preferred embodiment, the short duration high frequency pulses for the seed beam is produced by a very short Nd:YAG crystal and a λ/2 Pockels cell. 
     As compared with prior art high brightness lasers, we have achieved our very high brightness by reducing the pulse duration by about 2 or 3 orders of magnitude, from a few ns to 100 ps or less and by focusing on a very small spot, but we are able to simulate a much larger spot by very rapidly steering our high average power beam over the area we need.

This invention is a Continuation-in-Part application of Ser. No.08/295,283, filed Aug. 24, 1994 and now U.S. Pat. No. 5,434,875.

This invention relates to laser systems and in particular to high power,high brightness solid state laser systems.

BACKGROUND OF THE INVENTION

There is a growing need for reliable, economical X-ray sources for X-raylithography. It is known that X-ray sources can be produced byilluminating certain metals with very high brightness laser pulses.Required brightness levels are in the range of 10¹¹ to 10¹³ W/cm² forprojection lithography and 10¹³ to 10¹⁵ W/cm² for proximity lithography.To meet future commercial lithography needs, average laser powerrequirements are about 500 Watts for projection and 1000 Watts forproximity. In addition the lithography process needs call for an X-rayspot diameter of about a few 100 μm. Designing a laser to meet theserequirements involves solving several current problems. The first is thecorrection of aberrations due to thermal distortion and self focusing inthe laser rod. This problem is currently being dealt with by utilizing aStimulating Brillouin Scattering (SBS) cell to remove these aberrations.SBS cell materials perform efficiently for laser pulses of severalnanoseconds or greater. For nanosecond laser pulses, the energy neededto achieve the required brightness is 10 to 30 Joules per pulse and therepetition rate needed to achieve the required power is 100 to 30 hertz.This high pulse energy design creates two additional problems. Theamount of debris produced by nanosecond pulsed lasers focused on solidtargets, when operated at the required brightness and power levels, isunacceptable. (Studies done by Rutherford and CREOL indicate that thedebris level from metal targets is related to the pulse duration. Theshorter the pulse duration the lower the debris level.) There is aresearch program underway to reduce debris by using solid xenon as anX-ray target, but it is at a very early stage and costs are uncertain.The final problem is the cost of the X-ray lithography system.

Flash lamp pumped lasers involve high maintenance costs. Maintenancecosts can generally be reduced by pumping with diode lasers.Unfortunately, laser diodes required for the 10 joule per pulse 100 Hzlasers costs millions of dollars.

What is needed is a laser system that meets the needs of X-raylithography to provide 1) high average power and high brightness, 2) lowdebris levels and 3) low capital and maintenance cost.

SUMMARY OF THE INVENTION

The present invention provides a high average power, high brightnesssolid state laser system. We first produce a seed laser beam with ashort pulse duration and a pulse frequency in excess of 1,000 pulses persecond. A laser amplifier amplifies the seed beam to produce anamplified pulse laser beam which is tightly focused to produce pulseswith brightness levels in excess of 10¹¹ Watts/cm². Preferredembodiments produce an amplified pulse laser beam having an averagepower in the range of 1 kW, an average pulse frequency of 12,000 pulsesper second with pulses having brightness levels in excess of 10¹⁴Watts/cm² at a 20 μm diameter spot which is steered rapidly to simulatea larger spot size. These beams are useful in producing X-ray sourcesfor lithography.

In one preferred embodiment, the seed beam is produced in a mode lockedNd:YAG oscillator pumped by a diode array with the frequency of thepulses being reduced by an electro-optic modulator. In a secondpreferred embodiment, the seed beam is Q switched and includes a Pockelscell for cavity dumping. In a third preferred embodiment, the shortduration high frequency pulses for the seed beam is produced by cavitydumping of a short cavity resonator.

As compared with prior art high brightness lasers, we have achieved ourvery high brightness by reducing the pulse duration by about 2 or 3orders of magnitude, from a few ns to 100 ps or less and by focusing ona very small spot, but we are able to simulate a much larger spot byvery rapidly steering our high average power beam over the area we need.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing showing the principal features of a preferredembodiment of the present invention for producing high brightness pulselaser beams useful for X-ray lithography.

FIGS. 1A, 1B and 1C are qualitative representations of the pulse shapeat various stages of the embodiment shown in FIG. 1.

FIG. 2 is a drawing showing in greater detail a first portion of theembodiment of FIG. 1.

FIG. 3 is a drawing showing in greater detail a second portion of theembodiment shown in FIG. 1.

FIG. 4 is a drawing showing the amplifier pumping configuration usinglaser diodes for the embodiment shown in FIG. 1.

FIG. 5 is a drawing showing a cluster of tightly focused spots.

FIG. 6 is a drawing showing the details of the second preferred seedlaser system.

FIGS. 7A and B are drawings showing the details of the third preferredseed laser system. They show the effect of turning on a Pockels cell.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A preferred embodiment of the present invention can be described byreference to FIGS. 1, 2 and 3. As shown in FIG. 1, this embodimentconsists of a mode locked Nd:YAG laser oscillator 2, a pulse spacingselector 20, a beam expander 22, a polarizing beam splitter 26, a doublepass amplifier section 24 and a beam steering PZT 48 on which amplifierfolding mirror 38 is mounted. The output of amplifier 24 is focused to atiny spot on moving copper tape target 27. FIG. 2 describes the seedlaser laser section of the embodiment which is for producing very shortduration pulses at a very high repetition rate and FIGS. 3 and 4describe the amplifying section for amplifying the pulses to produce apulsed laser beam with an average power level of about 1 kW with pulsesat brightness levels in the range of 10¹⁴ W/cm² on spot sizes of about20 μm diameter. And finally, FIG. 5 shows the result of a beam steeringmechanism to generate a cluster of few 20 μm spots 52 over a 500 μmdiameter circular area on a metal target.

FIRST PREFERRED SEED LASER

FIG. 2 is a diagram of a Nd:YAG mode locked oscillator type laser device2. A Nd:YAG polished rod 4 (3 mm diameter and 2.5 cm long) islongitudinally pumped by a 5 bar impingement cooled laser diode array 6(SDL part number SDL3245-J5). The diode pump array is quasi-CW and ispreferably run at 20 percent duty factor (about 200 μs ON and 800 μsOFF) and 50 Watt average. The diode array wavelength is at 808 nm whichcorresponds to strong absorption in the Nd:YAG. The output of the pumpdiodes are collimated by an array of cylindrical micro-lenses 8. A fastfocusing lens 10 concentrates the pump light at the back end of Nd:YAGcrystal 4. The back surface of Nd:YAG crystal 4 has 5 m radius ofcurvature (convex) and is polished and coated for maximum reflection(about 99.8 percent) at 1064 nm (the lasing wavelength of the Nd:YAGlaser) and for at least 85 percent transmission for 808 nm (the pumpwavelength). The pump light is trapped in the laser rod via totalinternal reflection (similar to a fiber optics) for high pumpingefficiency. The front surface 12 of the Nd:YAG rod is cut at 2 degreesto avoid parasitic oscillations and AR coated for minimal insertionloses at 1064 nm. A Brewster cut acousto-optic mode locker 14 (BrimroseCorporation of America Model FSML-38-10-BR-1064) is placed next to apartially transmitting mirror 16 (output coupler) to actively force allthe longitudinal modes to be in phase each time they pass the modelocker. The RF carrier frequency (f) of the mode locker and the opticallength of the laser resonator (L) must relate as follows:

    f=c/4L

where c is the speed of light. In this embodiment, we drive mode locker14 with a 38 MHz RF driver 15. We provide a cavity length of about 6.5feet. Thus, a train of mode locked pulses at 76 MHz (due to the standingwaves that form in the acousto-optic cell) during the ON time will beobtained. The pulse duration will be about 100 ps with an energy perpulse of about 0.6 μJ. The time interval between pulses during the 200μs diode ON periods is about 13 ns. During each ON period we get about15,200 of these very short pulses. Then we have a dead time of about 800μs before the next series of 15,200 short pulses. We have 1,000 of theseOFF-ON sequences each second, so the result is an average of about15,200,000 short 100 ps pulses per second with the pulses coming inclumps of 15,200. A qualitative depiction of this pulse train is shownin FIG. 1A. The rapid series of pulses represent 15,200 pulses each withan energy of about 0.6 μJ per pulse spread over 200 μs and the spacerepresents an 800 μs dead time.

PULSE SPACING SELECTOR

As will be explained later, we will amplify each pulse from 0.6 μJ toabout 80 mJ; therefore, for an average power of 1 kW we need only 12,000pulse per seconds. To reduce the frequency of the pulses from 15.2million per second to 12 thousand per second, we place in the path ofbeam 18 exiting the seed laser a pulse spacing selector 20 as shown inFIG. 2. Pulse spacing selector 20 consists in this embodiment of anelectro-optic modulator such as Model 305 supplied by ConOptics. Thisunit will function as a fast shutter to pass light from the beam duringshort intervals (each interval having a duration of about 10 ns) at afrequency of 60,000 Hz. Since the pulses are coming into the selector at13 ns intervals, the pulse selector (synchronized with the beam) willpass a single pulse through each 10 ns window and block all otherpulses. At our frequency of 60,000 Hz, we will therefore have about 12pulses pass each 200 μs ON period. Since we have 1,000 of these ONperiods each second, we will get about 12,000 pulses per second. Thus,the output of pulse spacing selector 20 is a pulse train consisting ofclumps of about 12 pulses (each pulse having a duration of about 100 ps)spaced over 200 μs duration and these clumps of short pulses beingspaced at intervals of 1,000 per second. This is an average of 12,000pulses per second. To summarize, the output of pulse spacing selector isas follows:

    ______________________________________                                        Pulse duration    about 100 ps                                                Energy per pulse  0.6 μJ                                                   Peak power per pulse                                                                            6 kW                                                        Average frequency 12,000 pulses per second                                    Average power     7.2 mW                                                      Beam cross section                                                                              0.07 cm.sup.2                                               ______________________________________                                    

A qualitative depiction of this pulse train is shown graphically in FIG.1B. It is essentially the same as the train shown in FIG. 1A except thefrequency of the pulses during the ON period has been reduced by afactor of about 1,260.

BEAM EXPANDER

The output of the pulse spacing selector is expanded from a crosssection of about 0.07 cm² to a cross section of about 0.6 cm² with a 3:1beam expander 22 as shown in FIG. 2. Beam expander 22 consist of anappropriate combination of lenses or any of several commerciallyavailable beam expanders chosen for the 1064 nm Nd:YAG beam. The outputof beam expander 22 is directed to amplifier 24 as shown in FIG. 1.

AMPLIFIER

Elements of amplifier 24 for this preferred embodiment is shown ifFIG. 1. Two-pass amplification is shown in FIG. 3 and our preferredpumping configuration is shown in FIG. 4. The amplifier must boost theseed beam energy to the mJ/pulse level.

FIG. 3 shows the principal features of the amplifier other than theamplifier pumping equipment. As shown in FIG. 3, the linearly polarizedbeam 21 from beam expander passes through a thin film polarizing beamsplitter 26 and into a first Nd:YAG amplifier rod 28 then through a onehalf wavelength rotator 30 (to cancel thermally induced bi-refringence)then through a second Nd:YAG amplifier rod 32 then through quarter waveplate 34 (for shifting the polarization of the outgoing beam by 90degrees) and corrector lens 36 (for correcting the thermal lensing inthe Nd rods) and is reflected off high reflectivity (HR) mirror 38. Thebeam passes back through the elements of amplifier 24 for two passamplification and reflected off polarizing beam splitter 26 from whichthe beam is focused and directed to a metal target 27 as shown inFIG. 1. The amplifier pumping equipment is shown in FIG. 4. Thisequipment includes 64 modules of 50 Watt per module (nominal) laserdiode arrays 40 for a total of about 3 kW average power operating at 808nm wavelength, 20 percent duty factor (200 μs ON and 800 μs OFF). Inthis embodiment 16 sets are arranged (4 shown in the circumferentialdirection and 4 in the linear direction, not shown) as indicated in FIG.4. The output of the diode lasers are directed into the Nd:YAG rods 28and 32 with cylindrical lenses 42 and the rods are water cooled by waterjacket 44 as shown in FIG. 4.

Amplifier 24 provides a 1.3×10⁵ amplification of the input beam withgood preservation of the input beam (near diffraction limited beam, lessthan or equal to 2XDL). Thus, the output of Amplifier 24 is a pulsedlaser beam with the following characteristics:

    ______________________________________                                        Pulse duration      about 100 ps                                              Energy per pulse    80 mJ/pulse                                               Peak power per pulse                                                                              800 MW                                                    Average frequency   12,000 pulses per second                                  Average power       1 kW                                                      Beam diameter       9 mm                                                      Brightness (power/pulse)                                                                          2.5 × 10.sup.14  Watts/cm.sup.2                                         (20 μm dia. spot)                                      ______________________________________                                    

A qualitative description of the output of the amplifier is shown inFIG. 1C. It is substantially the same as the pulse train shown in FIG.1B except the pulses are amplified in energy by a factor of about133,000. We then focus the beam to a 20 ILm spot on the target.

Preferred sizes of the X-ray point source for proximity lithography isin the range of a few 100 μm (e.g., 500 μm)in diameter to about 1 mm indiameter. A 500 μm spot simulated from 20 μm diameter pulses is shown inFIG. 5. In order to achieve the proper spot size with the abovedescribed system, we have to hit the target at different spots (e.g.,multiple 20 μm spots 52 spread over a 500 μm area 50). This isaccomplished in this embodiment by mounting a mirror 38 on fast two-axisPZT 48 that steers the beam slightly over the required area as shown inFIG. 1

The above system provides very good X-ray conversion. However, asomewhat better X-ray conversion can be accomplished with higherfrequency beams. In a study by Lawrence Livermore laboratories, a 15percent conversion efficiency was observed when the laser wavelength was532 nm (doubled 1064 nm) versus 10 percent conversion efficiency for1064 nm. A doubling crystal (not shown) could be placed at the outputbeam from the amplifier in order to utilize the higher X-ray conversionefficiency at 532 nm.

SECOND PREFERRED SEED LASER

An additional approach to the seed laser (sub nanosecond pulse durationand higher than 1000 pulses per second seed laser) can be a Q-switchmode locked configuration or a Q-switch mode locked with cavity dumpingconfiguration (FIG. 6). Laser diode array 61 with μ-lenses 63 is focusedby lens 65 for end pumping of the Nd:YAG rod 67 as described in thepreferred embodiment. Polarizer beam splitter 71 reflects the laser Spolarization to form a folded cavity (resonator) which includes the modelocker 75 as previously described, an acousto optics Q-switch 73, and aλ/4 electro optics Pockels cell 69 such as 1041 FV-106 and 5046 driver(Fast Pulse Technologies) for cavity dumping.

Since Q-switch 73 spoils the resonator, the gain builds up in the Nd rodas it is being pumped by the laser diode array 61. As the Q-switch opensup, the mode locked pulses build up. The laser radiation is S polarizeddue to the high resonator Q in the S polarization, and is trapped in theresonator between the high reflectivity mirror 77 and the highreflectivity coating on the back surface of the Nd rod 67. As thetrapped mode locked pulse builds up to its maximum intensity, Pockelscell 69 turns on to give a λ/4 retardation. The mode locked pulse thatpropagates to the left undergoes twice λ/4 retardation which results ina P polarization after exiting the modulator to the right. Polarizerbeam splitter 71 (highly transmissive to P polarization) then transmitsthe pulse to provide the output seed beam 79. This seed laser can besubstituted for the oscillator 2 and the pulse spacing selector 20 shownin FIG. 1. The output is directed to beam expander 22 and the rest ofthe path of the beam is as shown in FIG. 1. Since the entire storedenergy in the Nd rod is used to generate the short pulse (about 100 ps)output beam, energies in the few mJ per pulse (vs. 0.6 μJ/p as describedin the first preferred seed laser) can be obtained from thisconfiguration.

The advantage of such a system is two fold: a) typical relaxationoscillation that takes place in free running solid state lasers (largeamplitude fluctuation) will not exist in the Q-switch mode, and b) theentire stored energy will convert to the desired mode locked pulses (nounused laser energy) which will result in much higher energy per pulseand therefore, lower gain or fewer passes through the amplifier will berequired.

THIRD PREFERRED SEED LASER

A drawing of a third preferred seed laser is shown is FIGS. 7A and B.This is a seed laser which generates a laser beam of sub nanosecondpulse duration at more than 1000 pulses per second. In this device alaser cavity is formed by high reflectivity mirror 91 and coating 83applied to the backside of Nd:YAG crystal 5. A polarizer beam splitter89 and a short and fast λ/2 Pockels cell 87 allow for cavity dumping.The oscillating beam 97 shown in FIG. 7A is P polarized in the plane ofthe paper due to the orientation of polarizer 89. When high voltage isapplied to Pockels cell 87, the cell will rotate the polarization of thebeam to the left by 90 degrees (perpendicular to the plane of thepaper). Whenever this happens, the polarizing beam splitter 89 willreflect the perpendicular polarization as shown in FIG. 7B. The resultis that a pulse, equal in length of twice the distance L (between thePockels cell 87 and the HR coatings 83), is directed to amplifier system96 which can be the system shown in FIG. 1 other than oscillator 2 andpulse spacing selector 20. It is easy to achieve L`s of 2 to 4 cm. Thetime duration of the pulse will be:

    t=2L/c

where c is the speed of light.

The pump beam can be CW or quasi CW. The repetition rate of the Pockelscell driver will determine the output pulse repetition rate and thelength L will determine the pulse duration. Since the crystal length canbe very small, the Pockels cell can be moved close to the coating 83 onits backside. Therefore, with L=1.5 cm, the pulse duration can bereduced to the range of about 100 ps.

While the above description contains many specificities, the readershould not construe these as limitations on the scope of the invention,but merely as exemplifications of preferred embodiments thereof. Thoseskilled in the art will envision many other possible variations whichare within its scope.

For example, with the first preferred seed laser, we could choose a muchshorter pulse duration than 100 ps. These could be obtained using apassive saturable absorber instead of the acousto-optic mode locker.With a saturable absorber we can get femtosecond pulses. It is ourbelief that the advantage of pulses in the 100 ps range is that we getsome heating of the plasma whereas the very very short pulses createsthe plasma but provides very little heating of it. The energy per pulseneeds to be in the range of 80 mJ/pulse when the objective lens is about12 cm from the target. A distance of at least 12 cm is recommended toavoid contaminating the lens with target material. However, if thisdistance is reduced the required energy per pulse could be reducedaccordingly because we could focus on a smaller spot. By doing so wecould reduce the energy per pulse requirement from about 80 mJ/pulse toas low as about 10 mJ/pulse.

The cost of laser diodes for pumping solid state lasers is primarilydominated by the peak power requirements and this determines the numberof diode bars. By operating the bars at a relatively high duty factor of20 percent and generating a large number of pulses per second, we canminimize the initial cost of the diode pumping system. For example, a 1kW system may require 3 kW average power from the pump diodes, a 20percent duty factor diode array system would require 15 kW peak power.Using 50 Watt peak bars at $700 per bar, the system would cost $210,000.In comparison, a 1 percent duty factor system would require 300 kW peakpower. The cost would be $4,000,000. Increasing the duty factor above 20percent, all the way to CW is feasible, but, balancing all factors(including system lifetime and complexity), we prefer a duty factor ofabout 20 percent. Persons skilled in the art will recognize that a flashlamp pumping system could replace the diode pumping system.

The first preferred seed beam pulse train frequency could be in therange of 10 MHz to 200 MHz or greater. With some compromise in theaverage power the number of pulses per second could be reduced down toabout 1,000 Hz.

The amplifier can be of slab or rod design. The solid state material canbe of a host material other than Nd:YAG. For example, Nd:YLF, Cr:LiSAF,Ti:S, etc. could be used. Amplification needed to boost the seed beam tothe mJ/pulse level can be satisfied by either high gain or multiplepasses. Up to eight passes can be done with passive components and muchhigher number of passes can be done in a regenerative amplifier. Thesteering mirror in the amplifier can be any reflecting element thatwould be appropriate to generate the cluster of spot sizes desired, suchas the 20 μm spots.

With respect to the first preferred embodiment, other devices could besubstituted for the electro-optic modulator for pulse spacing, such ascavity dumping or even an optical rotary interrupter. The pulse spacingdevices would in most applications remove a very large percentage of thepulses in the first preferred seed beam such as more than 99 percent asin the preferred embodiment described; however, We could imagineapplications where as the percentage remove might be as low as 80percent. Accordingly the reader is requested to determine the scope ofthe invention by the appended claims and their legal equivalents, andnot by the given examples.

We claim:
 1. A high average power, high brightness solid state pulselaser system comprising:a) a seed laser subsystem means for producing aseed pulse laser beam with a pulse frequency in excess of 1,000 pulsesper second each pulse having a duration of less than 1 ns, b) a laseramplifier means for amplifying said seed pulse laser beam to produce anamplified pulse laser beam comprising high frequency pulses, saidamplified pulse laser beam having an average power in excess of 10Watts, c) a focusing means for focusing said amplified pulse laser beamto a small spot size on a target, said spot size being small enough toproduce a brightness level in excess of 10¹¹ W/cm².
 2. A pulse lasersystem as in claim 1 wherein said seed laser subsystem means comprises apulse spacing selector means for removing more than 80 percent of thepulses in a laser beam to produce said first pulse laser beam.
 3. Apulse laser system as in claim 2 wherein said beam steering meanscomprises a PZT device attached to a mirror.
 4. A pulse laser system asin claim 2 wherein laser means comprises a mode locked laser oscillatorcomprising a mode locking means for producing a mode locked laser beam.5. A pulse laser system as in claim 4 wherein said mode locking means isan acousto-optic mode locker.
 6. A pulse laser system as in claim 2where in said pulse selector means comprises an electro-optic modulator.7. A pulse laser system as in claim 1 and further comprising a beamsteering means for rapidly steering said amplified pulse laser beamrelative to said target so as to simulate a spot size larger than saidsmall spot.
 8. A pulse laser system as in claim 1 wherein said beamsteering means comprises a means for moving said target relative to saidamplified pulse laser beam.
 9. A pulse laser system as in claim 1wherein said laser amplifier means comprises a multiple-pass Nd:YAGlaser amplifier pumped by a pumping means.
 10. A pulse laser device asin claim 9 wherein said pumping means comprises a plurality of laserdiode arrays.
 11. A pulse laser system as in claim 9 wherein saidpumping means comprises a flash lamp.
 12. A pulse laser system as inclaim 1 wherein said laser amplifier means comprises a Nd:YAG polishedrod pumped by a plurality of laser diode arrays.
 13. A pulse lasersystem as in claim 12 wherein said laser diode arrays are programmed tooperate CW.
 14. A pulse laser system as in claim 12 wherein saidplurality of laser diode arrays are programmed to operate at a dutyfactor of less than 100 percent.
 15. A pulse laser system as in claim 14wherein said duty factor is about 20 percent.
 16. A pulse laser systemas in claim 15 wherein said amplified pulse laser beam comprises aseries of periodically spaced high frequency pulses.
 17. A pulse lasersystem as in claim 1, and further comprising a target for the productionof X-rays upon illumination at said small spots.
 18. A pulse lasersystem as in claim 17 wherein said target is comprised of a metal.
 19. Apulse laser system as in claim 18 wherein said metals is chosen from agroup consisting of copper and iron.
 20. A pulse laser system as inclaim 1 wherein said seed laser subsystem defines a resonator andcomprises a mode locked laser comprising a Q switch.
 21. A pulse lasersystem as in claim 20 wherein said Q switch is a high gain Q switchhaving a gain in excess of 10 per pass and said resonator is a shortresonator shorter than 4 inches.
 22. A pulse laser system as in claim 20wherein said seed laser subsystem further comprises a cavity dumper. 23.A pulse laser system as in claim 1 wherein said seed laser subsystemcomprises a laser crystal having a reflective side, a λ/2 Pockels celland a polarizer beam splitter and said system defined by a length Lbeing the distance between said reflective side and said pockels cell.24. A pulse laser system as in claim 23 wherein said length L is nogreater than 4 cm.
 25. A pulse laser system as in claim 24 wherein saidlength L is no greater than 2 cm.
 26. A pulse laser system as in claim 1wherein said laser amplifier defines an operating wavelength and saidseed laser subsystem comprises a laser diode producing a pulsed laserbeam having a wavelength matched to the operating wavelength of saidlaser amplifier.
 27. A pulse laser system as in claim 1 and furthercomprising a frequency increasing means placed in the amplified beam forincreasing the frequency of the amplified beam.
 28. A high averagepower, high brightness solid state pulse laser system comprising:a) amode locked Nd:YAG laser oscillator for producing a first pulse laserbeam with a high pulse frequency, b) a pulse spacing selector means forremoving from said first pulse laser beam more than 80 percent of thepulses in said in said beam to produce a second pulse laser beamcomprising a series of periodically spaced high frequency pulses inexcess of 1,000 pulses per second, c) a multiple-pass, diode pumped,Nd:YAG laser amplifier means for amplifying said expanded pulse laserbeam to produce an amplified pulse laser beam with an average power inthe range of about 1 kW, said beam comprising high frequency pulses, e)a focusing means for focusing said amplified pulse laser beam to a smallspot size on a target, said spot size being small enough to produce abrightness level in excess of 10¹¹ W/cm².
 29. A pulse laser system as inclaim 28 and further comprising a beam steering means for rapidlysteering said amplified pulse laser beam relative to said target so asto simulate a spot size larger than said small spot.
 30. A pulse lasersystem as in claim 29 wherein said frequency increasing means is aharmonic generator.